Description of the Prior Art
It is known that vinyl acetate can be prepared in the gas phase from ethylene, acetic acid and oxygen; the supported catalysts used for this synthesis comprise palladium and an alkali metal element, preferably potassium. Further additives used are cadmium, gold or barium.
In the Pd/K/Au catalysts both noble metals are generally applied in the form of an impregnated layer on the support; they are produced by impregnation and subsequent precipitation of the metal salts by means of alkaline compounds (U.S. Pat. No. 4,048,096, U.S. Pat. No. 3,775,342).
In Pd/K/Ba catalysts the metal salts are applied by impregnation, spraying on, vapor deposition, dipping or precipitation (EP-A-0 519 436). The same methods are known for Pd/K/Cd catalysts (U.S. Pat. No. 4,902,823; U.S. Pat. No. 3,393,199, U.S. Pat. No. 4,668,819). Furthermore, the production of a Pd/K/Au or Pd/K/Cd surface impregnated catalyst is known, with a specific support material being washed with an acid prior to impregnation and being treated with a base after impregnation (EP-A-0 519 435).
The German Patent Application P 42 11 780.1 describes Pd/K/Au, Pd/K/Ba or Pd/K/Cd catalysts built up in the form of an outer layer by atomizing a solution of the corresponding metal salts by means of ultrasound and then, in a limited amount and over a limited time, applying this to the support particles and commencing the drying thereof in such a way that the catalytically active metal salts cannot penetrate into the core of the support particles, but only into an outer part having a greater or lesser thickness, the impregnated layer.